What is SPN 91 FMI 4 & How to Solve It?

What is SPN 91 FMI 4:

SPN 91 refers to the Accelerator Pedal Position Sensor 1, and FMI 4 means the voltage or signal is below the expected range.

This indicates that the throttle pedal sensor is not sending proper input to the ECM, which may result in poor throttle response or no acceleration at all.

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Why This Happens

• Broken or loose accelerator pedal wiring.
• Faulty pedal position sensor.
• Poor ECM ground or voltage supply.
• Pedal sensor connector corrosion.
• Calibration error or software issue.

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How to Fix It

1. Inspect the accelerator pedal wiring harness and plug.
2. Test sensor voltage and signal range with diagnostic tools.
3. Replace the pedal sensor if readings are out of range.
4. Check and clean ECM grounds and power feeds.
5. Clear the code and test pedal response after repairs.

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How to Avoid It Before It Happens

1. Check accelerator wiring and connectors at each scheduled service.
2. Protect the pedal sensor from excessive dust and moisture.
3. Calibrate throttle sensors after replacing pedals or ECM.
4. Train operators to report lagging or unresponsive acceleration early.
5. Use Clue to log drive behavior anomalies and get alerted before a throttle failure affects machine use.

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